1. Field of the Invention
This invention relates in one aspect to the production of an expanded polystyrene-bead product, and in particular, the invention relates to the use of such product as a lightweight aggregate in the production of light-weight concrete compositions.
2. Description of the Prior Art
The manufacture of expandable polystyrene beads has been practiced in the United States for about twenty years. Those skilled in the art are well familiar with the practices necessary to yield polystyrene beads which have a diameter on the order of 0.35 to 1.30 millimeters and also contain a suitable proportion of a blowing or expanding agent, such that when the unexpanded beads are subjected to heat, they expand from an original specific gravity on the order of 0.625 to a lower specific gravity such as 0.01 to 0.05. Apparatus for causing such expansion is known, such as that shown in Rodman U.S. Pat. No. 3,023,175. It is known, moreover, that the product from the operation for producing the unexpanded beads may be screened to produce various size-fractions from the output of the bead-producing operation. For example, there may be made a size fraction of unexpanded polystyrene beads which is 100 percent by weight between 1.29 and 0.35 millimeters in diameter and over 90 percent by weight of a diameter between 1.24 and 0.58 millimeters, i.e., a somewhat coarse product, with a great proportion of its particles typically having diameters between 0.65 and 1.0 millimeters. There may also be made a size fraction somewhat finer, i.e., having 100 percent by weight between 1.04 and 0.35 millimeters in diameter and having over 90 percent by weight of its particles between 0.9 and 0.4 millimeters, with a great portion of its particles thus ranging in diameter from 0.85 to 0.45 millimeters. Expanding the particles from a specific gravity of 0.625 to a specific gravity of 0.016 increases the diameter of the particles by a factor of about 3.4.
Although various other uses for polystyrene-bead products are known, we are particularly concerned with the use of such beads as a lightweight aggregate in the production of a lightweight concrete, such as the compositions disclosed in U.S. Pat. Nos. 3,214,393; 3,257,338; and 3,272,765. The lightweight concretes have, for example, specific gravities of 0.35 to 1.05, in comparison with values of 2.24 to 2.56 for conventional concrete. The polystyrene lightweight concretes do not have values as great in regard to compressive strength, giving rather typically 0.21 to 0.56 kilograms per square millimeter (300 to 800 pounds per square inch), where conventional concrete has a compressive strength of about 2.8 to 3.5 kilograms per square millimeter (4000 to 5000 pounds per square inch), but the polystyrene lightweight concretes are greatly superior in thermal insulating value, having efficients of thermal conductivity on the order of one-fourth to one-ninth as great as those of conventional concrete.
Lightweight concretes of low density, such as about 0.48 to 0.52 in specific gravity, have been used for roof fill. Material somewhat more dense (specific gravity of 0.52 to 0.64) is useful for curtain walls. Material of specific gravity 0.60 to 0.64 is useful as a sub-base for highways, railway rights-of-way, aircraft runways, and similar applications in which it is desirable to protect the sub-grade from repetitive freezing and thawing. Materials of specific gravity over 0.64 are useful as load-bearing walls and as fire walls.
The procedures for obtaining lightweight concrete having a desired specific gravity (within a broad, inexact range of target value plus or minus 0.03 or 0.05) are known to those skilled in the art. The polystyrene beads are expanded to the desired and proper degree, and then they are mixed with proportions of cement, water, sand (in at least most cases), and certain additives such as an air-entrainment agent (AEA) and a water-reducing admixture (WRA). It has been known that even at a particular density, the proportion of cement and sand may be varied somewhat; to obtain greater strength, it is usually possible, within limits, to use more cement and correspondingly less sand. In some instances, sand may be omitted altogether. On the other hand, whenever the strength is not needed, one may save money by using relatively more sand and less cement. The proportions of AEA used may also be varied, within limits, to influence the density of the product. In accordance with the prior art, however, it has not been possible to control or preordain in the specific gravity (density) of the product any more accurately than to the degree indicated above, that is, target value plus or minus 0.03 to 0.05. Such a range leaves considerable uncertainty in regard to the strength and thermal-insulating properties that will be obtained in the product lightweight concrete.
In accordance with practices known before the present invention, an attempt to make a concrete with a specific gravity of 0.640 grams per cubic centimeter (40 pounds per cubic foot) might yield one of only 0.576 (36 pounds per cubic foot). Such a concrete has a compressive strength of 20.4 to 23.9 kilograms per square centimeter (290 to 340 pounds per square inch) and a thermal conductivity of 0.144 to 0.180 watts per degree Kelvin-meter or 1.0 to 1.25 Btu/(hr) (ft..sup.2) (.degree.F./in.). On the other hand, it might yield a concrete of 0.688 grams per cubic centimeter (43 pounds per cubic foot). Such a concrete has a compressive strength of 24.6 to 38.7 kilograms per square centimeter (350 to 550 pounds per square inch) and a thermal conductivity of 0.173 to 0.216 watts per degree Kelvin-meter, or 1.2 to 1.5 Btu/(hr.) (ft..sup.2) (.degree.F./in.). Given this great variability, a designer had no choice but to use sufficient material to have the necessary strength and insulating values, assuming the most unfavourable case. A practice which permits the density and other properties to be accurately predetermined thus makes it possible to save at least 5 to 10 percent on the amount of material used, and in the case that the insulating value is the principal consideration, as much as 50 percent in some cases.
In the prior art of which we are aware it has been taught that the size of the polystyrene beads used in making lightweight concrete is not critical. The three above-mentioned United States patents each contain a specific statement to that effect, and they mention among them the use of expanded beads with diameters of 0.59 to 6 millimeters, with 3.175 millimeters being a typical diameter. This corresponds to the use of unexpanded beads having a diameter of 0.17 to 1.7 millimeters, with 0.94 millimeters being a typical value.